As the equipment which can measure a blood oxyecoia saturation in the blood of a subject, a pulse oximeter becomes popular. A pulse oximeter can measure the blood oxyecoia saturation in the blood of a subject by a non-invasive approach continuously. It is used widely from the newborn baby to the adult on the medical care scene. The pulse oximeter can be classified into the transparent type pulse oximeter and the reflective type pulse oximeter according to the difference of the principle of measurement and the structure. The pulse oximeter also can be classified into the one time use pulse oximeter and the re-usable pulse oximeter according to the intended-purpose and number of use times. Hereinafter, the re-usable and transparent type pulse oximeter is explained as an example of the conventional pulse oximeter.
The basic structure of the transparent type pulse oximeter comprises a red light LED, an infrared LED and a photo detector. The LED irradiates red light and an infrared light to the examined part of the subject, and the photo detector detects the light which penetrated without being absorbed through the biomedical tissue of the skin, the vein, the muscle, the bone and so on. It measures a blood oxyecoia saturation in the blood by the signal change. Because the oxyhemoglobin (the oxygenated hemoglobin) and the reduced hemoglobin (the deoxyhemoglobin) differ in the light absorption percentage, the transparent type pulse oximeter measures oxygen saturation percentage (SpO2) by irradiating two different wavelengths and observing light absorbed in the blood, and analyzing the static state and the component that changes in the time of the signal, and computing a rate with the hemoglobin quantity combining with oxygen in the arterial blood.
For example, portions such as a finger or an ear lobe are chosen as the examined portion for the pulse oximeter because these portions are easy to pinch by the measurement cavity in order to insert the examined portion of the subject in the measurement cavity between the LED and the photo detector to measure. When using the large-sized pulse oximeter which has a big measurement cavity on an infant, his arm or his leg is chosen as an examined portion. Here, when measuring the oxygen saturation percentage (SpO2) by the pulse oximeter, all the light received by the photo detector should be decreased in intensity by penetrating through the biomedical tissues of the examined portion. It is important to avoid receiving the irradiated light from the LED directly without the decrease in intensity by penetrating through biomedical tissues of the examined portion. If the irradiated light from the LED is received directly by the photo detector, the correct measurement of the oxygen saturation percentage (SpO2) cannot be performed.